Physiological factors affecting colonic motility

Eating and morning awakening appear to be the major stimuli in eliciting colonic motility. There is also evidence that the menstrual cycle and pregnancy cause disturbances in gastrointestinal function. Transit is delayed in the luteal phase of the cycle, i.e. when serum progesterone levels are highest, and thus progesterone may depress colonic motility28.

There are studies showing that no difference exists in colonic transit rates between men and women29 30, whilst other studies have reported a prolonged colon transit in women28 31 32. A study aimed specifically to resolve this dispute has failed to find any difference in colonic transit of radiopaque markers in 10 women in the follicular phase, in 10 women in the luteal phase of the menstrual cycle, in 5 women on oral contraceptives and in 11 males33. A wide variation in stool weight exists between subjects on a standard diet and response to wheat fibre34. This variation is significantly related to sex but not to age, height or weight. Stool weight in men (162±11 g. day-1 mean±se) was approximately double

Figure 7.3 The effect of age on transit thriugh the colon

that in women (83±11 g. day-1 mean±se) and could be explained entirely by differences in transit. The increase in stool weight with fibre was significantly related to dose with approximately 1 g of non-starch polysaccharides (the main component of dietary fibre) increasing stool weight by 5 g per day. Smaller increases in stool weight were seen in females, persons with initially low stool weights and small people. Faecal pH was lower in men than in women and was related to methane production. Methane producers had higher faecal pH than non-producers (7.06 compared to 6.65), lower stool weight (93±12 g. day-1 compared to 156 ±13 g. day-1) and slower transit times (84.6±11.7 h compared to 48.6±6.6 h). These studies show that, when on similar diets, women have much lower stool weights and slower transit times than men. Children appear to have similar colonic transit times to adults, although colonic transit is significantly delayed in the elderly (Figure 7.3).

The effect of stress on gastrointestinal transit remains controversial. Colonic motility can be increased by emotional stress35. It is generally recognized that abdominal pain increases and bowel function becomes irregular in patients with the irritable bowel syndrome in periods of emotional stress, and that the symptoms often improve as the anxiety diminishes.

Although exercise is often recommended as therapy for constipation, the relationship between gastrointestinal transit and exercise is unclear36. Certainly immobility leads to constipation, but increased exercise above the norm appears to have no effect. However, one study reported that moderate exercise decreases whole gut transit time from a mean of 51 hours to 37 hours when riding and 34 hours when jogging37.

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